Vapor pressure control in discharge lamps



Dec. 20, 1960 E. LEMMERS VAPOR PRESSURE CONTROL IN DISCHARGE LAMPS Filed April 18, 1958 u fll lnven tor: Eu ewe Lemmevs,

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2,965,789 Patented Dec. 26, 1960 nice Eugene Lernrners, Cleveland Heights, Ohio, assignor to general Electric Company, a corporation of New ork Filed Apr. 18, 1958, Ser. No. 729,425

7 Claims. (Cl. 313-204) This invention relates generally to mercury vapor pressure control in electric discharge lamps having elongated tubular envelopes and more particularly in noncircular lamps of grooved or kidney-shaped cross section.

In my copending application Serial No. 577,017, filed April 9, 1956, entitled Tubular Electric Lamps, and assigned to the same assignee as the instant application, now Patent 2,915,664, various forms of re-entrant cross section fluorescent lamps are disclosed. In one form, a transversely re-entrant groove extends longitudinally in continuous fashion along one side of the envelope. In another form, comparatively short sections of groove are provided alternating on opposite sides of the envelope. In yet another form of non-circular fluorescent lamp commercially available under the designation Power Groove, the longitudinal groove extends intermittently along'one side and is interrupted at several places by regions of circular cross section for increasing the strength of the envelope against implosion by reason of external atmospheric pressure. This form of lamp is disclosed and claimed in copending application in Serial No. 578,772 by Eugene Lemmers et al., filed April 17, 1956, entitled Tubular Lamp Envelopes, and likewise assigned to the same assignee as the present invention, now Patent 2,916,645.

A re-entrant or grooved cross section achieves a high ratio of perimeter to area of the cross section along with a relatively high implosion resistance. This is highly advantageous in resonance radiation lamps, for instance in fluorescent lamps utilizing the resonance radiation of mercury vapor at 2537 A. to excite a phosphor coated internally on the walls of the envelope in order to produce visible light, and achieves higher loading and lumen output per unit axial length at a given eificiency than heretofore possible.

There are at least three factors involved in the high light efliciency achieved with a grooved contour:

(1) A high electron speed is obtained simultaneously with reduced elastic collision losses due to more rapid diffusion of electrons, mercury ions and radiation quanta to the bulb walls, thereby improving the rate of delivery of 2537 A. radiation to the bulb walls.

(2) For a given lamp length and wattage, the lamp current is less and the lamp voltage higher than with lamps of equal periphery in a circular cross section, thereby reducting both the cathode losses and ballast losses.

(3) Proper mercury vapor pressure for most efficient generation of 2537 A. radiation is more easily maintained. The joining edge walls on either side of the groove are cooler than the remainder of the lamp and their temperature rises more slowly with increase in loading than the remainder of the cross section. This effect is due in part to constriction of the plasma or discharge causing it to draw away from the edge walls as the ,current is increased, the. consequence being reduced heating efiect.

At very high loadings, even the improved vapor pres sure regulating characteristics of grooved discharge lamps may be insufficient to maintain optimum mercury vapor pressure. All of the various solutions which have so far been proposed to this problem have definite disad vantages. The metal vapor isin equilibrium with an excess of nonvaporized metal and the vapor pressure is of course determined by the coolest point in the lamp. One proposal for achieving a cool point consists in blowing air at a suitable place on the lamp; this is generally cumbersome. Another proposal is to place heat shields wihin the lamp behind the cathodes in order to reduce heat radiation and convection flow of heated gas or vapor to the ends of the lamp. This does provide a cool spot in each end of the lamp which may be adequate for mercury vapor pressure control, but it shortens the effective length of the discharge space and results in unduly long dark ends for the lamp. Moreover since the mercury vapor tends to move longitudinally to the center of the lamp, it is desirable to have the cool spot or vapor pressure control center in that region so that equilibrium may be achieved more rapidly after the lamp is switched on.

An object of the invention is to provide a grooved discharge lamp having a new and improved vapor pressure regulating feature.

Another object of the invention is to provide a vapor pressure regulating center in a re-entrant groove lamp which is convenient and economical to manufacture and which does not require any special care of maintenance in operation of the lamp or in its handling in order to avoid breakage or damage.

In accordance with the invention, a metal vapor pressure regulating center is provided by forming the vitreous envelope of the lamp at places contiguous to one or more of the grooves to a contour which exludes or at least reduces the intensity of the discharge in selected regions. These regions as a result run cooler and serve as vapor pressure control centers. The special forms given to the glass envelope walls are so disposed that where they project outwardly from what would otherwise be the contour of the lamp, they are located entirely within one of the recessed grooves. Thus there are no projections beyond the general cylindrical configuration of the lamp so that the danger of breaking such projections during manufacture or susequent handling is eliminated.

In a preferred embodiment of the invention, one or more of the re-entrant grooves, preferably near the longitudinal center of the envelope, are provided with an upstanding fin extending outwardly from the bottom wall of the groove. The fin is relatively narrow so that the plasma or discharge does not penetrate into it and as a result it remains relatively cool. Being entirely located within the re-entrant groove, the fin does not project beyond the generally cylindrical con-figuration of the lamp so that it is not readily subject to breakage. .In another embodiment of the invention, a depression or indentation is provided on one side of the bottom or inner wall of the groove and projects inwardly into the envelope space. This causes the plasma or discharge to be excluded from the space between the indentation and the near edge Wall on one side of the groove. Accordingly that region of the, edge wall is cooled and serves as a mercuryvapor pressure control center.

For further objects and advantages, attention is now directed to the following detailed description of embodiments of the invention and to the accompanying drawings. The features of the invention believed to be novel will be more particularly pointed out in the appended claims.

In the. drawing: a

Fig. 1 is a pictorial view of a discharge lamp embodying a preferred form of the invention, portions of the envelope wall being removed to shorten the figure.

Fig. 2 is a transverse cross sectional view of the lamp in the plane 2-2 of Fig. 1.

Fig. 3 'is a side sectional view of the lamp taken in the plane 3-3 of Fig. 2.

Fig. 4 is a'transverse cross sectional view of another lamp embodying another formof the inventionJ Fig. 5 is a side sectional view of the lamp of Fig. 4 takenin the plane 5 -5.

Referringnow to Figs. 1 to 3, there is shown a fluores cent lamp of the low-pressure positive column type comprising an elongated vitreous envelope 1. The envelope is provided with circular or round tube ends 2, 2 which are annularly reduced or shouldered at their extremities for securing thereto bases 3, 3. As shown at the end of the lamp having the cut-away portion, an electrode mount or stem 4 is sealed into eachtube end. A pair of lead wires 5, 5 are sealed through the stem and support on their inward projections a filamentary cathode 6 while their outward projections are connected to the contact pins 7, 7 of the base. The cathode may consist of a coiled coil of tungsten wire provided with an overwind and coated with an activated mixture of alkaline earth oxides, such as the usual mixture comprising barium and strontium oxides.

The lamp contains an ionizable atmosphere including a starting gas or mixture of one or more of the inert rare gases of group of the periodic table at a low pressure,'for' instance argon at a pressure of 0.5 to millimeters of mercury or alternatively a mixture of argon and up to 20% neon in this range of pressure. The mercury vapor is provided by a small quantity of mercury exceeding in amount the quantity vaporized during operation of the lamp wherein the mercury vapor may exert a partial pressure in the range of 1 to 20 microns for optimum generation of mercury resonance radiation at 2537 A. A phosphor coating indicated at 8 on the inside of the envelope converts the 2357 A..radiation into visible light.

The envelope 1 is provided with spaced indentations or re-entrant portions 10, 11 on diametrically opposite sides giving a dimpled or crenelated appearance. The indentations 10, 11 may be considered to be short sections of a longitudinal groove alternating on opposite sides of the envelope, and the lamp is sometimes referred to as doublegrooved. The discharge cross section through the grooved sections of the lamp is in general kidney-shaped as illustrated in Fig. 2. The resulting configuration has a high implosion resistance and maintains good efficiency at high loadings by reason of the higher electron velocities and reduced elastic collision losses. These characteristics are due of course to the more rapid diffusion of photons, electrons, and mercury ions to the walls as previously mentioned. The sloping ends 12, 13 of adjacent grooves alternating on opposite sides of the envelopeare sufficiently close to form desirable constricted zones 14 between them. The discharge in these zones is constricted even as it is in the kidney-shaped cross section throughout the groove sections. Thus the discharge operates at a high efliciency even in these zones and furthermore the arc is lengthened by virtueof the zigzag path which it is constrained to follow. This further increases the. percentage of total lamp wattage going intorthe positive column relative to fixed electrode losses, thereby increasing the etficiency.

In accordance with the invention, there, is provided within one or more of the grooved sections 10, 11 an upstanding fin or ridge. In the illustrated embodiment, fins 15, 16 are shown in grooved sections 10' and 11'. Fin has side walls rising upwardly from bottom wall 17 of the groove section and merging on one side. into into the sloping end wall 12'. The fin does not extend all the way to the opposite end wall 12 of the groove but has a forward edge 18 sloping down to the bottom wall 17, and is symmetrically located along the medial line of the groove. The width of the fin is'subs tantially less than the least dimension of the discharge cross section defined by the envelope walls elsewhere throughout the lamp, with the result that the plasma or discharge does not spread into it to any appreciable extent. The fin in consequence runs much cooler than the remainder of the lamp and serves as a mercury vapor control center in which the excess mercury condenses. If desired the fin may extend the full length of the groove, that is from sloping end wall 12 to sloping end wall 12.

The fin may be formed by utilizing a split tool to press the groove section 10' wherein the fin is to be located. T he pressing tool and the general technique may be similar to that described in aforementioned application Serial No. 578,772 of Lemmers et a1. According to the technique described therein, the lamp envelope is selectively heated in the zones where the grooved panels are to be formed until the glass becomes suitably plastic, and then a plunger or pressing bar with suitable protuberances is pressed down against the envelope. The protuberances contact the glass in the heated zones and cause the envelope walls to fold inwardly in those zones thereby forming the reentrant cavities or grooves desired. One or more of the protuberances in the pressing bar is split or channeled at one end forming a recess in which the glass wall of the envelope remains upstanding to form the fin 15. Suction may be applied to the recess to assist in maintaining the glass wall upstanding and in orderto cause it to conform to the recess. The grooves on the opposite side of the envelope, and also additional fin 16 if desired, may be formed in like manner by rotating the envelope on its longitudinal axis and repeating the operation.

One or more of the groove sections located near the center of the lamp may be provided with a fin in order to provide several vapor pressure control centers whereby to accelerate the stabilization of the vapor pressure after the lamp is put in operation. Preferably at least one groove section on each side of the envelope should be provided with a It will then be immaterial which grooved side of the lamp faces downwardly or outwardly to either side when the lamp'is installed in a fixture. Convection air currents will maintain the'lowest temperature in one or the other, of the fins, whichever one happens to be more favorably located and that one will serve as the mercury vapor pressure controlicenter.

If desired, fin 15 on one side of the lamp and fin 16 on the other may be formed to merge into adjacent sloping end walls 12', 13' of the grooves in a back-to-back relationship, as illustrated in Figs. 1 and 3, for improved symmetry.

The provision of fins permits the lamps to be operated at substantially higher loadings than heretofore while yet maintaining the mercury vapor pressure therein at the desired optimum for most efiicient generation of 2537 A. radiation. For instance, in a T17 lamp 2% inches in diameter, the fins permit operation at 30 watts per foot at room temperature and provide a control temperature of approximately 40 C. 'for a mercury vapor pressure of approximately 6 microns, such being close to the optimum for this lamp.

Referring to Figs. 4 and 5, there is shown another embodiment of the invention in a re-entrant groove lamp wherein 'the groove 21 is disposed entirely along one side in discontinuous sections with upstanding portions or bridges '22' of generally circular section in between. In this form of lamp, the panels of the lamp where the groove provides'a kidney-shaped cross section are high efficiency regions as regards the production of light,

whereas thecircular sectioned bridge portions are low efiiciency regions but are necessary in order to achieve the desired implosion resistance. Whereas only a portion of the 'lamphas been shown in Fig. 5, the entire lamp mayin general be similar to that shown in Fig. 1 and previously described as comprising activated filamentary electrodes sealed into opposite ends and a. filling ,of fan ionizable medium comprising a rare gas and mercury fine itself at that point to the region between the indentation and the remote edge Wall'or rail 24 of the kidneyshaped cross section. 'The plasma assumes a generally lenticular cross section within this region as illustrated in dotted lines at 25. In the region indicated at 26 between indentation and near edge wall 24, the plasma or discharge does not penetrate. Accordingly this side of the lamp and particularly the edge wall or rail at 24 operate at a substantially lower temperature and serve as an effective mercury vapor pressure control center. Actual tests have shown that the provision of an indentation as illustrated in Figs. 4 and 5 may cause a drop in the temperature of the edge of the lamp of as much as 15 C. under what would otherwise be the temperature of the edge walls on either side of the groove. The region between the indentation and the near edge wall thus serves as a vapor pressure control center allowing operation of the lamp at optimum vapor pressure for substantially higher loadings.

While certain specific embodiments of the invention have been illustrated and described in detail, these are intended as illustrative and not as limitative of the invention. The scope of the invention is to be determined by the following claims which are intended to cover any modifications coming within its true spirit and scope.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A low pressure electric discharge lamp comprising an elongated vitreous envelope having electrodes sealed into opposite ends and containing an ionizable medium including an inert starting gas and a metal vapor in equilibrium with an excess of non-vaporized metal, said vitreous envelope being entirely contained within a generally cylindrical boundary and having substantial longitudinally grooved portions, said envelope having at least one deformation in the wall contour of the groove within one of said grooved portions and located entirely within said cylindrical boundary, said deformation providing a constricted zone within the envelope which causes the discharge plasma to be substantially excluded therefrom, said zone thereby operating at a cooler temperature than the remainder of said envelope and serving as a metal vapor pressure control center.

2. A low pressure electric discharge lamp comprising an elongated vitreous envelope having electrodes sealed into opposite ends and containing an ionizable medium including an inert starting gas and mercury vapor in equilibrium with an excess of non-vaporized mercury, said vitreous envelope being entirely contained within a generally cylindrical boundary and having substantial longitudinally grooved portions of generally kidneyshaped cross section, said envelope having at least one deformation in the wall contour of the groove within one of said groove sections and located entirely within said cylindrical boundary, said deformation providing a constricted zone within the envelope and causing the discharge plasma to be substantially excluded therefrom, said zone thereby operating at a cooler temperature than the remainder of said envelope and serving as a mercury vapor pressure control center.

3. A low pressure electric discharge lamp comprising an elongated vitreous envelope having electrodes sealed into opposite ends and containing an ionizable medium including an inert starting gas and mercury vapor in equilibrium with an excess thereof in liquid form, said vitreous envelope being entirely disposed within a generally cylindrical boundary and having substantial por= tions kidney-shaped in cross section by means of longitudinally extending re-entrant groove sections, and at least one upstanding fin portion in a groove section, said fin portion extending outwardly from the bottom Wall of the groove toward said cylindrical boundary and being relatively narrow in transverse dimension in order to exclude the electric discharge plasma therefrom, said fin thereby providing a zone operating at a cooler temperature than the remainder of the envelope and serving as a mercury vapor pressure control center.

4. A low pressure electric discharge lamp comprising an elongated vitreous envelope having electrodes sealed into opposite ends and containing an ionizable medium including an inert starting gas and mercury vapor in equilibrium with an excess thereof in liquid form, said vitreous envelope being entirely disposed within a generally cylindrical boundary and having substantial portions kidney-shaped in cross section by means of a series of relatively short longitudinal grooves alternating on opposite sides and having sloping end walls, and at least one upstanding fin portion in one of said grooves, said fin portion extending outwardly from the bottom wall of the groove toward said cylindrical boundary and sloping to said bottom wall at one end and merging into the sloping end wall of the groove at the other end, said fin being relatively narrow in transverse dimension in order to exclude the electric discharge plasma therefrom, said fin thereby providing a zone operating at a cooler temperature than the remainder of the envelope and serving as a mercury vapor pressure control center.

5. A low pressure electric discharge lamp comprising an elongated vitreous envelope having electrodes sealed into opposite ends and containing an ionizable medium including an inert starting gas and mercury vapor in equilibrium with an excess thereof in liquid form, said vitreous envelope being entirely disposed within a generally cylindrical boundary and having substantial portions kidneyshaped in cross section by means of a series of relatively short longitudinal grooves alternating on opposite sides and having sloping end walls, and an upstanding fin portion in at least two of said grooves disposed on opposite sides of said envelope said fin portions extending outwardly from the bottom wall of the groove toward said cylindrical boundary and sloping to said bottom wall at one end and merging into the sloping end of the groove at the other end, said fins being relatively narrow in transverse dimension in order to exclude the electric discharge plasma therefrom, said fins thereby providing at least one zone operating at a cooler temperature than the remainder of the envelope and serving as a mercury vapor pressure control center irrespective of the position of the grooves in mounting the lamp.

6. A low pressure electric discharge lamp comprising an elongated vitreous envelope having electrodes sealed into opposite ends and containing an ionizable medium including an inert starting gas and mercury vapor in equilibrium with an excess thereof in liquid form, said vitreous envelope being entirely disposed within a generally cylindrical boundary and having substantial portions kidneyshaped in cross section by means of longitudinally extending re-entrant groove sections, and at least one additional indentation in the bottom wall of a groove section, said indentation being directed inwardly into the discharge space and located to one side of the medial line of the groove section whereby to provide a constricted zone between the indentation and the near edge wall on one side of the groove from which the discharge plasma is substantially excluded, said zone thereby operating at a cooler temperature than the remainder of the envelope and serving as a mercury vapor pressure control center.

7. A low pressure electric discharge lamp comprising an elongated vitreous envelope having electrodes sealed into opposite ends and containing an ionizable medium including an inert starting gas and mercury vapor in equione side of the medial line of the groove section whereby 10 to provide a constricted zone between the indentation and the near edge wall on one side of the groove from which the discharge plasma is substantially excluded, said zone thereby operating at a cooler temperature than the remainder of the envelope and serving as a mercury vapor pressure control center.

References Cited in the file of' this patent UNITED STATES PATENTS 2,020,726 Gaidies Nov. 12, 1935 2,190,009 Boucher Feb. 13, 1940 2,213,245 Germer Sept. 3, 1940 2,317,265 Foerste Apr. 20, 1943 2,501,375 Breadner Mar. 21, 1950 FOREIGN PATENTS 802,714 France June 13, 1936 861,799 France Nov. 4, 1940 

